In the gas production industry, there is a need to efficiently separate oxygen from oxygen containing streams at ambient or sub-ambient temperatures.
In cryogenic air separation, nitrogen (N2), oxygen (O2) and argon (Ar) are separated based on their boiling points and relative volatilities. A first cryogenic column provides a rough separation of the three main components of air: N2 (78%), O2 (21%), and Ar (1%). A side stream is removed and sent to a second column known as the side arm column or crude argon column. This stream is called “crude” because it exits this side arm column at only about 95% argon. The conventional methods of further purifying this crude argon are limited to: “Deoxo” purification, getter bed technologies, and additional distillation. The Deoxo process reacts controlled amounts of hydrogen with the oxygen in the argon stream to remove the oxygen. Because the reaction of hydrogen and oxygen generates significant heat, this process can be dangerous if not controlled properly. Getter beds only function at lower oxygen concentrations by reacting oxygen with copper catalyst to form copper oxide. When high purity argon is desired, a third distillation column can be used to further concentrate it. Unfortunately, these distillation columns require upwards of 200 stages due to the similarity in boiling points of O2 and Ar and are less economical than is desired.
To achieve a kinetic separation of O2 from either N2 or Ar by an adsorption mechanism, an adsorbent structure must be developed with very specific pore dimensions. The Lennard-Jones 6-12 kinetic diameter of Ar (3.40 Å) is smaller than that of O2 (3.46 Å), but O2 is not a spherical molecule and has a minimum molecular dimension that could be exploited. The symbol Å represents the Ångström, a unit of length, which is defined as 10−10 meters. Adsorption mechanisms suggest that the minimum molecular dimension is the limiting factor for kinetic exclusion. With the proper orientation, O2 should diffuse into a pore with an effective diameter of 2.8 Å. Argon, a spherical atom, will have a constant diameter of 3.4 Å. This 0.6 Å difference in diameters is the key sensitivity that an O2 selective adsorbent must demonstrate to achieve a kinetic separation between oxygen and argon. With such an adsorbent, a process could be derived that purifies crude argon from the cryogenic air separation process in a safer and more economical manner and removes O2 from argon much more rapidly and efficiently.
Compared to the conventional, very elaborate methods of recovering argon from a crude argon stream, a PSA process provides a simple and effective alternative for argon purification and recovery. No hydrogen or additional cryogenic stages are required. However, removing oxygen and nitrogen simultaneously from crude argon stream requires either two separate PSA stages or a PSA column comprising two layers of adsorbents with different utility characteristics.
Kumar et al. in U.S. Pat. No. 4,477,265, discloses a two stage VSA (vacuum swing adsorption) process for argon purification. The two layers of adsorbents for oxygen and nitrogen removal are in two separated stages. The two stages are connected in series. This allows the process to be more flexible. For example, it permits possible bed interactions even within a stage and using different number of beds in different stages. In one preferred embodiment, three beds are in fact used in the first stage for nitrogen removal using a nitrogen equilibrium selective adsorbent. Two beds are in the second stage for oxygen removal using an oxygen rate selective adsorbent. The basic cycle steps include adsorption, evacuation, and pressurization. Also, argon recovery is low, and recycling the waste stream, still containing considerable amount of argon, back to cryogenic unit is necessary for additional recovery. Recycling of VSA waste stream back to the cryogenic plant makes the air separation unit more complex and a VSA option less attractive. It is important to note that a VSA instead of a PSA process is used in the layer bed configuration.
Pressure swing adsorption (PSA) processes comprising several layers of adsorbents are known in the open literature. However, the arrangement of the two layers of adsorbents with different characteristics in the same PSA column requires careful consideration. For example, one layer may be composed of an adsorbent with kinetic selectivity to the contaminant gas, where the product gas has very slow diffusion kinetics relative to the contaminant gas. Another layer may be composed of an adsorbent, which separates gases based on differences in equilibrium capacities, where the contaminant gas is more adsorbed than the product gas. The application WO2008072215A2 discloses such a PSA process for upgrading natural gas. The novelty described in this application is the non-conventional arrangement of an adsorbent with smaller capacity and performing a kinetic separation, followed by an adsorbent performing the separation by differences in the adsorption equilibrium of the species. Note that only one component is removed by the two layers.
U.S. Pat. No. 5,730,003 describes a hybrid process where crude argon produced in a cryogenic distillation plant is processed in a 2-bed pressure swing adsorption (PSA) unit to produce 99.999% argon. If the crude argon contains significant amount of nitrogen in addition to oxygen, the patent reports to include a nitrogen selective adsorbent in a layer separate from the oxygen selective layer. Carbon molecular sieve (CMS), type A zeolite, clinoptilolite, and the adsorbents disclosed in U.S. Pat. No. 5,294,418 are used as an oxygen selective layer. As a nitrogen selective layer, adsorbents such as CaA, type X zeolite (LiX or NaX), and zeolite of type A & X containing mixed cations selected from groups I and II of the periodic table (LiNaX) are mentioned. The layering preference, PSA feed temperature and regeneration conditions are not reported. In the description of the PSA process, an optional vacuum pump is incorporated. It is not clear whether the adsorption process operates under pressure swing or vacuum swing mode for simultaneous removal of oxygen and nitrogen from argon stream using the layered bed.
Carbon molecular sieves (CMSs) have been developed that selectively adsorb O2 over N2 based on the smaller kinetic diameter of the O2 molecule, see e.g. Yang, R. T., Gas Separation by Adsorption Processes, Butterworths, Boston, 1987. More recently, in S. U. Rege and R. T. Yang, Adsorption, 2000, Vol. 6, 15-22; and U.S. Pat. No. 7,501,009 to Graham, et al., CMSs are proposed for removal of O2 from Ar by kinetic separation.
Nevertheless, use of CMSs for the purification of crude N2 or Ar presents several problems, including, but not limited to, a potential for combustion if CMS dust is returned to the cryogenic distillation tower and low recovery due to variable pore size distribution. Zeolites are porous aluminosilicates, which are non-combustible, more stable towards oxygen exposure than CMSs, and contain well controlled pore sizes based on their highly crystalline structure. Thus, they have the potential to overcome many of these drawbacks.
An inherent problem with many kinetic PSA processes for the purification of crude N2 or Ar utilizing either zeolite or CMS adsorbent is low recovery of the desired N2 or Ar product due to low utilization of the full capacity of adsorbent. This arises because where feed step must be stopped well before adsorbent saturation to avoid contamination of the primary product with a high level of the impurity. In such case, it is necessary to recycle the PSA waste stream, still containing significant amount of argon or nitrogen, back to the cryogenic air separation unit for additional recovery. An improvement in kinetic selectivity would enhance the bed utilization, and thus the final argon recovery.
A RHO zeolite has a symmetric, three-dimensional pore structure containing channels with openings made up of two 8-membered oxygen rings. The nominal ring diameter or opening is 3.6 Å. This is close to the target pore dimensions, mentioned above, for the kinetic separation of O2 from Ar and N2, and N2 from Ar vide supra. This pore dimension could also be useful in the separation of CO2 from methane.
RHO zeolites require the presence of large cesium extra-framework cations as the structure directing agent during synthesis, and do not occur naturally. They were first prepared in 1973 by Robson and coworkers (Advances in Chemistry Series, 1973, 121, 106.). This initial synthesis used no additional organic templating agents and produced RHO materials with a ratio of Si to Al atoms equal to 3.2, hereafter specified by the shorthand RHO(3.2). More recently, RHO zeolites have been synthesized by Chatelain and coworkers using 18-crown-6 as a templating agent (Microporous Materials, 1995, 4, 231). The templated method gives highly crystalline RHO with Si/Al=3.9 to 4.5, i.e., RHO(3.9) to RHO(4.5). The preponderance of structural work with RHO has been carried out with RHO(3.2) and RHO(3.9) materials. RHO compositions with Si/Al>4.7 have been reported by Mitsubishi Chemical in WO15020014A1 through a mixing modification of the Chatelain procedure. They specifically claim copper and iron exchanged RHO materials at these higher Si/Al ratios for NOx reduction applications, but mention other transition metals, including zinc, in their background. The use of any of these materials in air separation is not mentioned. Lower Si/Al compositions with only copper, iron, or zinc are not taught, and it is not obvious that they would be stable, based on the greater basicity of the lower Si/Al RHO materials. In U.S. Pat. No. 5,944,876, Corbin teaches of fully and partially cadmium (Cd) exchanged RHO zeolites, with Si/Al>3, including RHO compositions with at least 1 Cd2+ cation per unit cell, with an assortment of other cations, including Zn2+. Because of the size of the Cd2+ cations, these compositions require at least one cation, namely the Cd2+ cation, to reside in an 8-ring position. Corbin does not teach how to prepare Cd RHO materials with a balance of Zn2+ cations. It is not obvious that RHO compounds with 3<Si/Al<5 would be stable if <one cation per unit cell was required to occupy one of the 8-ring positions, due to the greater basicity of lower Si/Al RHO compositions.
Higher Si/Al RHO materials with Si/Al>5 have been prepared by use of excessive templating agent (Ke Q., Sun T., Cheng H., Chen H., Liu X., Wei X., Wang S. Chem Asian J., 2017, 12, 1043.)
The as-prepared, hydrated, RHO zeolites crystallize with a centrosymmetric body centered cubic (bcc) structure, but it has been shown that this structure can undergo rather large distortions to lower symmetry upon dehydration and depending on the type of extra-framework cation substitution. The distortion, which can be observed as a large unit cell contraction, is largely driven by the distortion of the RHO 8-rings. Corbin and coworkers have shown that the undistorted, essentially circular rings of the proton exchanged RHO can distort to highly elliptical rings on exchange of small, high charge density cations such as Ca2+ and Li+ (Journal of the American Chemical Society, 1990, 112, 4821).
In principal, this distortion mechanism could be used to tune the ring size, shape, or diameter to selectively adsorb certain gases over others by size exclusion processes. This mechanism has been exploited by Corma and coworkers (Chemical Communications, 2012, 48(2), 215) and Wright and coworkers (Journal of the American Chemical Society, 2012, 134, 17628) to achieve large equilibrium selectivity for CO2 adsorption over methane. In U.S. Pat. No. 7,169,212, Corbin describes the use of mixed-cation RHO zeolite, Li7.1Na1.93Cs0.3Al11.7Si36.3O96, also specified here by the shorthand notations of Li7.1Na1.93Cs0.3RHO(3.1) for separation of oxygen from nitrogen in a PSA process. In this last case, though the kinetic selectivity for oxygen vs. nitrogen adsorption is extremely high, the RHO 8-ring size has been made sufficiently small that the uptake of even the smaller gas, oxygen, is extremely slow and is not practical for standard PSA applications.
Several researchers mention equilibrium-based separations of O2 from Ar, but there are few rapid kinetic separations reported. Most of these kinetic separations use carbon molecular sieves (CMS), which, by size-selectivity, are able to adsorb O2 at rates about 30× faster than Ar at ambient temperatures (U.S. Pat. No. 6,500,235 and S. U. Rege and R. T. Yang, Adsorption, 2000, Vol. 6, 15-22). U.S. Pat. No. 6,500,235 also mentions the use of a transition metal containing metal organic framework (MOF). Both materials show relatively slow uptake and, because these materials contain organic components, the adsorption process waste streams must be filtered if the streams are to be recycled back to a cryogenic plant. CMS materials are typically not suitable for separation of N2 from Ar, because they have essentially no equilibrium selectivity and limited kinetic selectivity between these two gases.
Only a few zeolite materials have been reported for the kinetic separation of O2 from N2 or Ar at ambient temperatures. S. Kuznicki, B. Dunn, E Eyring, and D. Hunter (Separation Science and Technology, 2009, 44:7, pp 1604-1620) report the kinetic separation of O2 from Ar using the Ba exchanged titanosilicate, Ba-RPZ-3. Rates appear to be slow for this material and the kinetic selectivity of O2/Ar is less than 10. S. Farooq (Gas Separations and Purification, Vol. 9, No. 3, pp 205-212) describes the possible use of a modified 4 A material, RS-10, from UOP. This is the only commercial zeolite based material which, at ambient temperatures, shows kinetic selectivity for O2 adsorption over N2 and Ar comparable to CMSs, at similar O2 uptake rates to the CMS materials.
D. W. Breck (Zeolite Molecular Sieves, Robert E. Krieger Publishing Co., 1984) describes the use of zeolite 4 A at sub-ambient temperatures for kinetic O2/N2 and O2/Ar separations.
Kovak et al. in U.S. Pat. No. 5,159,816 mention the use of a list of zeolite adsorbents including 5 A, 4 A, mordenite, 13X, and chabazite for removing N2 and O2 from Ar in a cryogenic TSA process.
Therefore, it remains desirable to develop adsorption processes that are useful for the separation of O2 from mixtures, such as those containing also N2 and/or Ar, that have at least comparable recovery to existing processes using CMS materials, but that are capable of operating at much higher adsorption and desorption rates. It also remains desirable to develop processes that are useful also for the removal of low levels of N2 from Ar.